1. Field of the Invention
The present invention relates to a liquid discharge head substrate and a head unit.
2. Description of the Related Art
A contact pad (a connection terminal) works as an electric contact between a recording apparatus and a head unit capable of being mounted to the recoding apparatus. The contact pad can be touched by a user who has not carried out a static elimination processing when the user attaches/detaches the liquid discharge head. In such a case, a surge voltage by a static electricity discharge enters internal elements of a liquid discharge head from a terminal and can break the internal elements, so that the liquid discharge head is required to have a countermeasure for the static electricity discharge. U.S. Pat. No. 6,945,622 discusses a configuration in which a protection diode is provided as a static electricity protection circuit in an input terminal provided on a liquid discharge head substrate.
The liquid discharge head substrate mounted in the liquid discharge head is produced using semiconductor production processing. To cut down on the cost by increasing numbers of products which can be produced from one piece of wafer, downsizing of the head is required, so that reduction of an area for wiring is advancing. Therefore, in the protection diode, the reducing an area of the liquid discharge head has advanced by providing the wiring with a laminated structure.
An example of configurations of a circuit of a liquid discharge head substrate and a wiring layer is illustrated in FIGS. 8A, 8B, 8C, and 8D. The liquid discharge head has a configuration in which a protection diode is provided in an external terminal as a static electricity protection circuit. FIG. 8A illustrates a block diagram of the protection diode. The external terminal 101 electrically connecting to an outside is provided at an end of a first wiring 22 connecting to an inverter circuit 301. The first wiring 22 is further connected to a second wiring 55 via a first protection diode 103 and a third wiring 66 via a second protection diode 104.
FIG. 8B is a top view illustrating an example in which the protection diode in the X part illustrated in FIG. 8A is downsized by laminating a plurality of wirings and provided. In the first wiring 22, a first lower conductive layer 118 and a first upper conductive layer 102 are laminated and connected via a through hole 1001 provided in the second insulation layer 115 made of SiO. The first lower conductive layer 118 and the upper conductive layer 102 are made of a conductive material such as aluminum. In this structure, the first lower conductive layer 118 has an equal potential to the first upper conductive layer 102. A second lower conductive layer 105, which forms the second wiring 55, connects to a potential connecting to a large capacity power supply (the potential can be an almost same potential used in a signal input from the terminal 101: hereinafter referred to as a power supply potential). A third lower conductive layer 106, which forms the third wiring 66, is connected to a substrate potential. Further, on a lower side of the second lower conductive layer 105 and the third lower conductive layer 106, a first insulation layer 114 and a thermally-oxidized layer 113 are provided. The first insulation layer 114 is made of boron phosphorus silicon glass (BPSG), and used as an insulation layer and a heat accumulation layer. The thermally-oxidized layer 113 is formed by oxidizing a substrate 109 made of silicon. The second lower conductive layer 105 and the third lower conductive layer 106 are connected to the first protection diode 103 and the second protection diode 104, which are formed in the silicon substrate, via a plurality of through holes 1003 provided in the first insulation layer 114.
FIG. 8C is a cross-sectional view of a line C-C′ of the first protection diode 103 in FIG. 8B, which is connected to the power supply potential. In a p-type substrate 109, a n-type well region 110, a n-type (n+) impurity diffusion region 111, p-type (p+) impurity diffusion region 112, and the thermally-oxidized layer 113 are formed. The first insulation layer 114 made of BPSG is formed on the above described layers. The through hole 1003 is formed in the thermally-oxidized layer 113 and the first insulation layer 114. The impurity diffusion region 112 and the first lower conductive layer 118 are connected to each other, and the impurity diffusion region 111 and the second lower conductive layer 105 are connected to each other respectively, so that the first protection diode 103 is formed.
FIG. 8D is a cross sectional view of a line D-D′ of the second protection diode 104 in FIG. 8B, which is connected to the substrate potential. In a p-type substrate 109, a p-type well region 120, a n-type (+n) impurity diffusion region 111, a p-type (+p) impurity diffusion region 112, and the thermally-oxidized layer 113 are formed. The first insulation layer 114 made of BPSG is formed on the above described layers. The through hole 1003 is formed in the thermally-oxidized layer 113 and the first insulation layer 114. The impurity diffusion region 112 and the third lower conductive layer 106 are connected to each other, and the impurity diffusion region 111 and the first lower conductive layer 118 are connected to each other respectively, so that the second protection diode 104 is formed.
With this configuration, when a surge voltage caused by static electricity is applied from the contact pad of the head unit, a surge current flows in the terminal of the liquid discharge head from the contact pad. Further, the surge current flows from the terminal to the upper conductive layer 102, and flows from the upper conductive layer 102 to the second lower conductive layer 105 through the first protection diode 103 or to the third lower conductive layer 106 through the second protection diode 104. With the configuration, the surge current caused by the static electricity can be prevented from flowing in an inside of the inverter circuit 301, so that dielectric breakdown of a switching element can be prevented.
In this case, in the protection diode, it is required that insulation between the upper conductive layer 102, and the second lower conductive layer 105 and the third lower conductive layer 106 is provided by the second insulation layer 115. More specifically, in an area Y of the second insulation layer 115, the insulation between the upper conductive layer 102 and the second lower conductive layer 105, and the insulation between the upper conductive layer 102 and the third lower conductive layer 106 need to be secured. The upper conductive layer 102 has an equal potential to the surge voltage, the second lower conductive layer 105 has the power supply potential and the third lower conductive layer 106 has the substrate potential.
However, since the second insulation layer 115 is sandwiched between the upper conductive layer 102, and the second lower conductive layer 105 or the third lower conductive layer 106, which have different potentials from each other, dielectric breakdown can arise. Particularly, in a step part (a concavo-convex part) of the through hole 1003 of the lower conductive layer, that is, a thickness of the second insulation layer 115 in the end part of the first insulation layer 114 is thinner than the second insulation layer 115 in a flat part. Thus, the dielectric breakdown of the second insulation layer 115 in the area Y can occur depending on a size of the surge voltage.
Particularly, in the liquid discharge head, which discharges a liquid utilizing heat generated by an energy generation element, there is a close relationship between a thickness of the layers of the thermally-oxidized layer 113, the first insulation layer 114, and the second insulation layer 115, and discharge characteristics of the liquid discharge head, such as a heat accumulation property and heat irradiation property. Thus, it is actually difficult to make the second insulation layer 115 thick enough to prevent the dielectric breakdown, when a compatibility with the discharge characteristics of liquid discharge head is considered.